Vascular stent having increased radiopacity and method for making same

ABSTRACT

A stent for use in a patient&#39;s blood vessel to maintain the patency of the vessel contains strategically located radiopaque material. The strategic placement of the radiopaque material in the core structure of the stent functions to enhance the resolution of the stent under fluoroscopy. The initial part of the process includes forming a groove in a piece of tube stock and securing radiopaque material into the groove by press fitting or diffusion bonding. After the securing method, a layer of material can be sputtered coated over the only radiopaque material or over the entire stent. Finally, a pattern of struts and splines is cut into the tube composite to form the stent.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to improvements in increasing theradiopacity of stents and improvements in their method of manufacture,and more particularly, to a stent and a method of manufacture whereradiopaque material is secured to strategic location(s) on the stent toimprove visibility of the stent under fluoroscopy.

[0002] Generally, stents are expandable endoprosthesis devices which areadapted to be implanted into a patient's body lumen to maintain thepatency of the vessel. Stents are especially well-suited for thetreatment of atherosclerotic stenosis in blood vessels. These devicesare typically implanted into blood vessels by a delivery catheter whichis inserted at an easily accessible location and then advanced throughthe patient's vasculature to the deployment site. The stent is initiallymaintained in a radially compressed or collapsed state to enable it tobe maneuvered through the lumen. Once in position, the stent is usuallydeployed either automatically by the removal of a restraint, or activelyby the inflation of an expandable member, such as balloon, about whichthe stent is mounted on the delivery catheter.

[0003] The stent must be able to satisfy a number of mechanicalrequirements. First, the stent must be capable of withstanding thestructural loads that are imposed by the vessel walls. In addition tohaving adequate radial strength, the stent should be longitudinallyflexible to allow it to be maneuvered through a vascular path and toenable it to conform to a deployment site which may not be linear or mayflex. The stent material must allow the stent to undergo expansion whichtypically requires substantial deformation of localized portions of thestent's structure. Once expanded, the stent must maintain its size andshape throughout its service life despite the various forces that maycome to bear thereon, including the cyclic loading induced by thebeating heart. Finally, the stent must be biocompatible so as not totrigger any adverse vascular responses.

[0004] Fluoroscopy is typically used to facilitate the precise placementof a stent as well as to verify the position of a stent within thepatient throughout the stent's service life. The use of radiopaquematerials in the construction of the stent allows for its directvisualization. Accordingly, different patterns and contents ofradiopactivity have different effects on the direct visualization. Forexample, when a physician views a completely radiopaque stent underfluoroscopy, he/she will likely see an unclear and amorphous shape thatextends outside the dimensions of the actual stent. The opposite willalso be true where the stent possesses little radiopacity. In terms offluoroscopic visibility, the optimal stent should be visible in a clearand detailed form without shape blurring. To date, no single materialhas been identified that simultaneously satisfies all requirementsinherent in an optimal stent application. Those materials that dosatisfy the mechanical requirements are either insufficiently orexcessively radiopaque and/or are not adequately proven to bebiocompatible in a vascular setting. Thus, simply constructing a stentwhich exhibits optimal radiopacity wholly out of a single material isnot currently a viable option. A number of different approaches have,however, been employed wherein different materials are combined in aneffort to render a mechanically sound and biocompatible stent to bevisualized by a fluoroscope.

[0005] One procedure frequently used for accomplishing fluoroscopicvisibility is through physical attachment of radiopaque markers to thestent. Conventional radiopaque markers, however, have a number oflimitations. When attached to a stent, such markers may project from thesurface of the stent, thereby exhibiting a departure from the idealprofile of the stent. Depending on their specific location, the markermay either project inwardly tending to disrupt blood flow or outwardlytending to traumatize the walls of the blood vessel. Additionally, whenthe metal used for the stent structure comes in contact with the metalused for the radiopaque marker, galvanic corrosion may occur. Thiscorrosion may lead to separation of the metals and thereaftercontamination of the blood stream with radiopaque material.Additionally, the radiopaque material may come into direct contact withliving tissue which may be problematic, particularly if the material isnot biocompatible.

[0006] Stents can also be marked by plating selected portions thereofwith radiopaque material. A number of disadvantages with this approachare apparent. Because the radiopaque material comes into direct contactwith living tissue, there can be a sizeable amount of tissue exposure.Additionally, when the stent is expanded and certain portions undergosubstantial deformation, there is a risk that cracks will form in theplating which can separate from the underlying substrate. This sideeffect has the potential for creating jagged edges on the stent whichmay inflict trauma on the vessel wall or cause turbulence in the bloodflow thereby inducing thrombogenesis. Moreover, once the underlyingstructural material becomes exposed, interfaces between the twodisparate metals become subject to the same type of galvanic corrosionas mentioned above. Further, should the plating pattern cover less thanall of the stent's surfaces, the margins between the plated and unplatedregions are all subject to galvanic corrosion.

[0007] As a further alternative, a stent structure can be formed from asandwich of structural and radiopaque materials. Tubes of suitablematerials can be cold-drawn and heat treated to create a multi-layeredtubing which can be cut into a stent. Struts and spines are formed inthe multi layered tubing by cutting an appropriate pattern of voids intothe tubing using well known techniques in the art. While this approachdoes provide a stent that has enhanced radiopacity and techniquesfulfills necessary mechanical requirements, the thin cross section ofthe radiopaque material is usually exposed along the edges of all cutlines. The biocompatiblity of the radiopaque material remains an issueand more significantly, a sizeable area is created which is subject togalvanic corrosion. Any cuts in the sandwich structure cause twodisparate metal interfaces, i.e. the juncture between the outerstructural layer and the central radiopaque layer, as well the juncturebetween the central radiopaque layer and the inner structural layer, tobecome exposed to blood and tissue along the entire lengths of thesecuts.

[0008] A stent configuration that overcomes the shortcomings inherent inpreviously known devices is therefore required. More specifically, astent is needed that provides radiopaque properties enabling clearvisibility under fluoroscopy and mechanical properties consistent forreliable and safe use.

[0009] A method of manufacturing the above mentioned stent configurationis also necessary. More specifically, a method is needed that combinesthe prerequisites of biocompatible materials and fluoroscopy into anadvantageous method of manufacture.

SUMMARY OF INVENTION

[0010] The present invention provides a stent and method for manufacturewhich overcomes some of the shortcomings of previously known stents andmethods of manufacturing stents. Most importantly, the stent has highresolution when viewed under fluoroscopy due to strategic placement ofradiopaque material along the stent. The stent also fulfills therequirements of having sufficient flexibility, structural integrity andbiocompatiblity, and being safe for deployment into a patient'svasculature.

[0011] Unique to the stent described herein is an advantageouslyselected pattern of radiopaque material formed on the stent. Compared toconventional stents, which are frequently obscured under fluoroscopy,the pattern of radiopaque material in the stent described herein leadsto improved visibility under fluoroscopy. Unique to the process is amethod of forming selected patterns of securely mounted radiopaquematerial on a stent substrate. Compared to some conventional processesof forming stents where radiopaque material is simply layered onto thestent structure, the process described herein utilizes a process ofplacing select patterns of grooves into the stent substrate. Thisgrooving process allows precise placement and strong retention ofradiopaque material into strategic locations of the stent.

[0012] The material employed for said underlying structure is selectedfor its structural and mechanical properties and may be the same generalmaterial used to make conventional stents. One preferred material is316L stainless steel, although other materials such as nickel-titanium,cobalt based alloys, Nitinol and other types of stainless steel can beused. Such materials, when used in the 0.002″ to 0.003″ thickness, as istypical for stent applications, are often difficult to visualizefluoroscopically.

[0013] The grooving process is the first operation to be performed on apiece of tube stock. In this process, a pattern of groove(s), preferablyeither rings or lines, is cut in the tube stock. The grooves should bestrategically placed at targeted locations along the length of thetubing to obtain the desired radiopacity. The stent may employ one or amultiple number of grooves depending on radiopacity requirements. Informing the groove(s), an instrument, such as a conventional Swiss screwmachine can be used. Alternative machines can also be used to performthe same grooving operation.

[0014] After the grooving process is performed on the tube stock,radiopaque material is inserted into the groove(s) by eitherpress-fitting, diffusion bonding or laser bonding. One preferredradiopaque material is gold, although other radiopaque materials such asplatinum, tantalum, iridium, or their alloys can also be used. Whenpress-fit, the shapes of the strip(s) must be in close conformance withthe shape of the groove(s) while being slightly larger than the size ofthe groove(s). The radiopaque strip(s) are combined with the groove(s)in the tube such that the difference in sizing causes the two metals tolock together in an interference fit. The interference fit insures astrong and long lasting bond between the two materials. When theradiopaque strip(s) are diffusion bonded, an entirely different processof attachment can be employed. In the diffusion bonding procedure, avacuum is drawn and the entire assembly (tubing with radiopaque materialinserted into the groove(s)) is heated to near the particular diffusionbonding temperature with the bonding surfaces still exposed to thevacuum environment. Thereafter, the bonding surfaces are brought intocontact with very moderate pressure and maintained at a temperature andpressure sufficient for diffusion bonding. The assembly is then cooled,resulting in a substantially unitary diffusion bonded structure.

[0015] While not mandatory for this process, stainless steel can beapplied over the radiopaque sections of the tube to promotebiocompatibility and structural integrity. Additionally, the stainlesssteel coating can act to protect the radiopaque and structural materialsfrom galvanically corroding. The stainless steel, preferably 316L can beapplied by a sputtering procedure, a method of depositing a metallicfilm through the use of electric discharge. Sputter coating machines arecommercially available and capable of applying an extremely even coatingof material to a workpiece. The tubing may be rotated in front of anozzle, the nozzle may be rotated about the tubing or a nozzle thatcompletely surrounds the tubing may be employed to apply the sputtercoating. While the preferred material for the sputtering is 316Lstainless steel, other suitable material can be used also. Additionally,if a higher degree of structural rigidity is sought, the material can besputtered across the entire length of the tube to a sufficient thicknesssuch that the structural integrity of the stent is significantlyincreased.

[0016] After the tube has been processed as described above, a procedurefor cutting the tube can be initiated. In this procedure, for example,the tubing is first placed in a rotatable fixture inside a cuttingmachine, where it is positioned relative to a laser. According tomachine encoded instructions, the tubing is rotated and movedlongitudinally relative to the laser. The laser selectively removes thematerial from the tubing by ablation and a pattern is cut into the tube.The laser cut provides a desired pattern of voids defining struts andspines which allows the stent to expand in an even manner, in accordancewith well known and well established procedures. Thereafter, the stentsare subjected to the standard industry practices of electro-polishingand possibly annealing. Another biocompatible outer layer could also beapplied to the stent.

[0017] The above and other objects and advantages of this invention willbe apparent from the following more detailed description when taken inconjunction with the accompanying drawings of exemplary embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018]FIG. 1 is a perspective view of the tube stock with one particularpattern of grooves cut into the tube in a ring-shaped pattern, constantin diameter, and spaced apart longitudinally.

[0019]FIG. 2 is a perspective view of the tube stock with radiopaquematerial inserted into the grooves and annularly bonded in acomplementary fit.

[0020]FIG. 3 is a perspective view of the tube stock with radiopaquematerial inserted into grooves with sputtered material entirely coveringthe markers.

[0021]FIG. 4 is a perspective view of a portion of a stent which can becut from the composite tube stock illustrated in FIGS. 1-4 and embodyingfeatures of the present invention.

[0022]FIG. 5 is a cross sectional view of a representative strut of astent made in accordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0023] A radiopaque stent and the process of forming said stent isdescribed herein. Unique to the stent is an advantageously selectedpattern of radiopaque material which is affixed to the stent. Comparedto conventional stents which are frequently obscured under flouroscopy,the pattern of radiopaque material in the stent of the present inventionallows an easily discernable view of the stent under fluoroscopy. Uniqueto the process of manufacturing the stent is a method of formingselected patterns of radiopacity within the stent. Compared to someconventional processes whereby radiopaque material is layered onto stentstructures, the method of the present invention includes a groovingprocess which allows for precise location of radiopaque material intothe core of the stent.

[0024] Referring now to the drawings, wherein like numerals indicatelike elements, a representation stent 10 made in accordance with thepresent invention is shown in FIG. 4. The stent 10 can be made in anynumber of different strut patterns, depending on the particularapplication for the stent. The stent 10 is representative of just onedesign which can be used to form the various struts and spines.Referring now to FIG. 1, a piece of tube stock 11 to be used for theunderlying structure of the stent is shown. The material employed forsaid underlying structure is selected for its structural and mechanicalproperties and may be the same material from which conventional stentsare made. One preferred material is 316L stainless steel, although othermaterials such as nickel-titanium, cobalt based alloys, Nitinol andother types of stainless steel can be used. Such materials, when used inthe 0.002″ to 0.003″ thickness which is typical for stent applications,are often difficult to properly visualize fluoroscopically.

[0025] As shown in FIG. 1, a series of grooves 12 may be formed in tubestock 11 by a Swiss screw machine operation, or other machinery. Onepreferred pattern of groove(s) is either ring(s) or line(s), but anyother pattern can be used. Such grooves can be placed at targetedlocations along the length of the tubing in order to obtain the desiredradiopactivity along the stent. In addition, the number of grooves canvary according to the stent's application. For instance, if highradiopacity is required, a plurality of grooves can be formed along thetube stock. If low radiopacity is required, as little as one groove canbe formed along the tube stock. While one preferred machine to form thegrooves is a conventional Swiss screw machine, other machines can beused to perform the same grooving operation.

[0026] Referring now to FIG. 2, the tube stock 11 may incorporateradiopaque material 13 inserted into the grooves 12. The radiopaquematerial can be formed into strips which are placed into the grooves.One preferred radiopaque material is gold, although other radiopaquematerials such but not limited to, platinum, tantalum, iridium, or theiralloys can also be used. The strips are preferably inserted into thegrooves by either a press-fit, diffusion bonding or laser bonding. Whenthe strips are press-fit into the grooves, their shape must be in closeconformance with, but slightly larger than, the width of the groove. Thepress fit ensures an interference between the radiopaque strips and thetube material which secures both materials together in a strong,long-lasting bond. If the strips are diffusion bonded to the tube, adifferent type of process is necessary. First, a vacuum is drawn and theassembly, which includes the tube and radiopaque strips placed in thegrooves, is heated to near diffusion bonding temperature with thebonding surfaces still exposed to the vacuum environment. Thereafter,the bonding surfaces are brought into contact with very moderatepressure and are maintained at a temperature and pressure sufficient fordiffusion bonding. The assembly is then cooled, resulting in asubstantially unitary diffusion bonded structure. The advantageouslyselected patterns of radiopacity will allow precise orientation ordegree of expansion to be discerned by inspection of the fluoroscopicimage when the stent is completed.

[0027] Referring now to FIG. 3, the tubing 11 may incorporate radiopaquematerial 13 in the form described above with the addition of a thinlayer of stainless steel 14 covering the radiopaque material. Thestainless steel is applied to the tubing by sputtering, a method ofdepositing a metallic film through the use of electric discharge.Sputter coating machines are commercially available and capable ofapplying an extremely even coating of material to a workpiece. Inpractice, the tubing may be rotated in front of a nozzle, the nozzle maybe rotated about the tubing or a nozzle that completely surrounds thetubing may be employed to apply the sputter coating. While one preferredmaterial for the sputtering is 316L stainless steel, other suitablematerial can be also be used. In addition to securing the radiopaquestrip(s) to the tubing, the sputtered layer of metal can function toprevent galvanic corrosion and strengthen the entire stent. In thisregard, the material can be sputtered to a sufficient thickness overselected regions of the tube 11 or over the entire tube such that thestructural integrity of the stent is significantly increased.

[0028] Referring now to FIG. 4, the composite radiopaque tubing 11 isillustrated with a substantial amount of material removed to passing thestruts and spines of the stent 10. In the material removal procedure,the tubing is placed in a rotatable fixture of a cutting machine whereit is positioned relative to a laser. The machine rotates and moves thetubing longitudinally relative to the laser, in accordance with machineencoded instructions. The laser selectively removes the material fromthe tubing by ablation and a pattern 14 is cut into the tube. The lasercut provides a desired pattern of voids defining struts and spines whileleaving both the radiopaque strips 13 and sputtered stainless steelcoating 14 in strategic locations. The tube is therefore cut into thediscrete pattern of the finished stent. Further details on how thetubing can be cut by a laser are found in U.S. Pat. Nos. 5,759,192(Saunders) and 5,780,807 (Saunders), which have been assigned toAdvanced Cardiovascular Systems, Inc. and are incorporated herein byreference in their entirely.

[0029]FIG. 4 illustrates a portion of a representative stent 10 wherethe radiopaque strips 13 and the sputtered coating 14 are integral tothe stent 10 and accompany the cut patterns 14. FIG. 5 shows a crosssectional view of a representative strut of the stent 10 made inaccordance with the present invention which has a strategically locatedradiopaque material and a sputtered coating 14 affixed thereto. In apreferred embodiment, the placement of the radiopaque material on thestock tubing can be coordinated with the particular pattern of strutsand spines which will be cut into the tube to ensure that the radiopaquematerial is completely surrounded by the tubing material once the tubeis cut. Therefore, there should be no edges on a strut or spine whichexposes the layer of radiopaque material directly to blood or tissue.The layer of coating which is sputtered onto the radiopaque materialshould complete the encapsulation of the radiopaque material. In thismanner, the layers of material should not be exposed to possibleelements which can cause galvanic corrosion or the layers to delaminate.Thereafter the stent is subjected to the standard industry practices ofelectro-polishing and possibly annealing. A biocompatable outer layercould also be added to the stent surface.

[0030] It should be appreciated that the radiopaque material may not becompletely surrounded by tubing material and the layer of sputtercoating in all instances. It is possible that some radiopaque materialmay be exposed on the sides of the stent struts after the tubing is cut.However, exposure of the radiopaque material can be kept at a minimum tohelp prevent galvanic corrosion from occurring and the risk of cracksforming along the struts. It is still possible to sputter an additionallayer of coating onto the stent after it has been cut to assure that noedges of the struts expose radiopaque material directly to blood andtissue. In this manner, the radiopaque material on the stent can befully encapsulated. Alternatively, a stent manufactured in accordancewith the present invention can be made by first placing the radiopaquematerial into the grooves formed on the tubing and then cutting thestruts and spines of the stent prior to any coating of the tubing.Thereafter, once the struts and spines of the stent have been properlyformed, the thin layer of coating could then be placed on the stent tofully encapsulate the radiopaque material.

[0031] An advantage of the stent, and the method for manufacturedescribed above, lies in the resolution of the stent under fluoroscopy.As previously mentioned, the high resolution is due to the strategicallyplaced radiopaque strips inserted into grooves formed in the stent. Thebenefits of the stent are immediately apparent in practice where aphysician, who views the stent under fluoroscopy, will clearly see thesize and location of the stent in the vessel of the patient. The clearview of the stent enables the physician to perform his functionefficiently and safely without the worry of incorrectly approximatingthe size or location of the stent.

[0032] While a particular form of the invention has been illustrated anddescribed, it will also be apparent to those skilled in the art thatvarious modifications can be made without departing from the spirit andscope of the invention. More specifically, it should be clear that thepresent invention is not limited to tubular type stents nor is itlimited to any particular method of forming the underlying stentstructure. Additionally, the invention is not limited to the use of anyparticular materials in either the core, radiopaque coating orencapsulating layer nor is it intended to be limited to any particularcoating or application method. Accordingly, it is not intended that theinvention be limited except by the appended claims.

What is claimed:
 1. A method for forming a vascular stent havingincreased radiopacity, comprising the steps of: selecting a tube ofstructural material; forming at least one groove along the tube;inserting radiopaque material into the groove; securing said radiopaquematerial to the tube; cutting the tube into a particular pattern to formspines and struts of a stent.
 2. The method of claim 1, wherein: thetube of structural material is formed of a biocompatible materialselected from the group consisting of stainless steel, nickel-titaniumalloys, and cobalt-based alloys.
 3. The method of claim 1, wherein: thestep of forming the groove comprises forming one continuous groove. 4.The method of claim 1, wherein: the radiopaque material is formed in astrip and the step of inserting the radiopaque material comprisesinserting the strip into the groove.
 5. The method of claim 1, wherein:the method of claim 1, wherein the step of securing the radiopaquematerial includes the step of laser binding the radiopaque material intothe groove.
 6. The method of claim 4, wherein: the step of securing thestrip includes press-fitting the strip into the groove.
 7. The method ofclaim 4, wherein: the step of securing the strip includes the step ofdiffusion bonding the strip into the groove.
 8. The method of claim 4,wherein: the step of forming the groove comprises forming a plurality ofgrooves.
 9. The method of claim 8, wherein: a plurality of strips ofradiopaque material are inserted into the plurality of grooves.
 10. Themethod of claim 1, wherein: the step of securing the radiopaque materialinto the groove includes press-fitting the material into the groove. 11.The method of claim 1, wherein: the step of securing the radiopaquematerial includes the step of diffusion bonding the material into thegroove.
 12. The method of claim 1 wherein: the grooves are formed as aplurality of rings.
 13. The method of claim 1, wherein: the cutting stepis performed by a laser.
 14. The method of claim 1, wherein: the step offorming the grooves includes forming them on the exterior of the tube.15. The method of claim 1, further including the step of: sputtercoating a layer of metal over the exterior surface of the radiopaquematerial.
 16. The method of claim 1, wherein: the step of forming thegrooves includes forming them on the exterior of the tube in a selectedpattern.
 17. The method of claim 16, wherein: the step of cutting thetube is performed in a pattern which selectively places radiopaquematerial on particular struts or spines of the finished stent.
 18. Astent having enhanced radiopacity, comprising: a tubular sectionconstructed of at least one segment of a tube; the segment having atleast one groove formed therein; a radiopaque material disposed withinthe groove; and means for securing the material within the groove. 19.The stent of claim 18, wherein: the means for securing the radiopaquematerial in the groove includes sputter coating a layer of metal overthe radiopaque material.
 20. The stent of claim 18, wherein: the segmentincludes a target area; and the groove is formed in selective patternsin the target area.
 21. The stent of claim 18, wherein: the segment isformed with one groove along a longitudinal line and the radiopaquematerial is located in the groove.
 22. The stent of claim 18 wherein:the means for securing the radiopaque material within the grooveincludes press fitting the material into the groove.
 23. The stent ofclaim 18, wherein: the means for securing the radiopaque material withinthe groove includes diffusion bonding the material into the groove. 24.The stent of claim 18, wherein: the segment is formed with a pluralityof grooves and the radiopaque material is formed in strips which areplaced within the grooves.
 25. The stent of claim 18 wherein: the meansfor securing the radiopaque material within the grooves includes pressfitting the strips of radiopaque material into the grooves.
 26. Thestent of claim 18, wherein: the means for securing the radiopaquematerial within the grooves includes diffusion bonding the strips ofradiopaque material into the grooves.
 27. The stent of claim 18,wherein: the means for securing the radiopaque material in the groovesincludes sputter coating a layer of metal over the strips of radiopaquematerial.